Because the players who PAY THEM to play the game ask for it.
So?
Blizzard isn't forcing us to play, if we don't like what we offer then we can stop playing. Unless they want to do that they ain't gonna do stuff.
Although I do agree with it. And I wish they'd have more customization for gear. IMO, I'd love to level with, say, a Defias Rapier that's been upgraded slowly over time. Not heirlooms, either. Just, your weapon can sort of "level up" with you, and you can slowly give it more stats/damage/dps overall. That way we can have customized weapons, rather than everybody having the same weapons at the end of the game.
Along with that, make end-game gear a lot harder to obtain. I'd like to actually work for my gear, not do heroic-after-heroic to get Justice Points for it.